Method and device in a telecommunications system

ABSTRACT

In a method in a central node of a cellular radio network the central node is adapted to deciding if a mobile station connected to the cellular radio network via a first radio access technology controlled by the central node is to be moved to another radio access technology when a circuit switched call of the mobile station is ended. It is assumed that the cellular radio network is adapted to use a fallback to circuit switched procedure. First it is decided that a CS call for a particular mobile station has ended. Then in it is checked if the CS call was the result of a fallback to CS procedure. The central node then decides if the mobile station is to be moved to another radio access technology based at least partly on the, i.e. if the CS call was a result of a CS fallback procedure.

TECHNICAL FIELD

The present disclosure relates to a method and device in atelecommunication system, in particular to a method and device forproviding a preferred distribution of idle User Equipments (UE:s)between different Radio Access Technologies (RATs), and also methods anddevices for providing information to the RATs in order to obtain thepreferred distribution.

BACKGROUND

Mobile Circuit Switched (CS) services based on Global System for Mobilecommunications (GSM) and Wideband Code Division Multiple Access (WCDMA)radio access allow obtaining telecommunication services with a singlesubscription in almost all countries of the world. Also today, thenumber of CS subscribers is still growing rapidly, boosted by the rollout of mobile CS services in dense population countries such as Indiaand China. In addition the classical Mobile Switching Center (MSC)architecture is changed into a so-called softswitch solution whichallows using packet transport infrastructure for mobile CS services.

The third generation partnership project (3GPP) has completed the workitem “Evolved UTRA and UTRAN” that defines a Long-Term Evolution (LTE)concept to assure competitiveness of 3GPP-based access technology. Itwas preceded by an extensive evaluation phase of possible features andtechniques in the Radio Access Network (RAN) workgroups that concludedthat the agreed system concepts can meet most of the requirements and nosignificant issue was identified in terms of feasibility.

In parallel to the RAN standardization 3GPP has also driven a SystemArchitecture Evolution (SAE) work item to develop an evolved corenetwork. The SAE core network is made up of core nodes, which arefurther, according to an Ericsson proposal split, into Control Plane(Mobility Management Entity, MME) and User Plane Gateway (ServingGateway, S-GW and PDN Gateway, PDN GW) nodes. The resulting networkarchitecture is shown in FIG. 1. Thus, FIG. 1 depicts nodes andinterfaces in an LTE radio network and also a terminal (UE) connectedthereto.

Common to both LTE and SAE is that only a Packet Switched (PS) domainhas been specified, i.e. all services are to be supported via thisdomain. GSM and WCDMA however provide both PS and CS accesssimultaneously.

A solution called “CS Fallback” has been standardized and the stage 2solution is defined in 3GPP Technical Specification TS 23.272 and FIG. 2shows the “CS Fallback” architecture. In FIG. 2 the nodes and relatedinterface between the nodes of a cellular radio network employing a CSFallback solution is depicted. The nodes are typically implemented usinghardware and software adapted to perform the different tasks associatedwith a respective node and also with communication means for digitallyconnecting with other nodes using the depicted interfaces.

The main principle of “CS Fallback” is that the terminal, i.e. themobile station (MS) or User Equipment (UE) is performing normal SAEMobility Management (MM) procedures towards the MME while camping on LTEradio access. The MME registers the terminal in the MSC Server (MSC-S)for CS based services using the SGs-interface. When a page for CSservices is received in the MSC-S it is forwarded to the terminal viathe MME (using the SGs-interface) and then the terminal performsfallback to GSM EDGE Radio Access Network (GERAN) or UniversalTerrestrial Radio Access Network (UTRAN) and responds to paging via theselected RAN. Similar behavior applies for CS services originating inthe terminal (mobile station). When these are triggered and the terminalis camping on LTE access (evolved UTRAN, E-UTRAN), the terminal performsfallback to GERAN or UTRAN and triggers initiation of the CS servicethere. Different solutions have been discussed for the “fallback”mechanism, such as PS Handover and inter-Radio Access Technology (RAT)cell change order (possibly with NACC, Network Assisted Cell Change)

FIG. 3 shows the case of a Mobile Station initiating a call in ActiveMode and when PS Handover (HO) is supported. It is further described insection 6.2 of 3GPP TS 23.272, and one particular step, step 3, isfurther described in 3GPP TS 23.401 and depicted in FIG. 4.

Once the terminal (MS/UE) is done with the CS service in CS domain, itmay return back to E-UTRAN using existing mechanisms or may be kept in2G or 3G coverage, i.e. provided radio access via GERAN or UTRAN.

In idle mode, a mobile station normally performs cell reselection usinginformation received in system information from the network. The networkmay however decide to change the cell reselection behavior forindividual terminals. For example, the system information might be setto favor LTE before WCDMA and GSM but the network might detect that aparticular user is better served in GSM. In such a case, the network canchange the cell reselection parameters for that terminal so it will stayin GSM while in idle mode. It is however typically important andbeneficial to keep a preferred distribution of UE:s in idle mode betweenGSM/WCDMA systems and LTE.

When a CS connection to an E-UTRAN capable MS/UE being connected to a 2Gor 3G network is about to be terminated/released, the Base StationController/Radio Network Controller (BSC/RNC) may decide whether:

-   -   To send the MS/UE to E-UTRAN using explicit signaling, also        known as release in GERAN or Radio resource Control (RRC)        Connection Release with Redirect in UTRAN or by including the        “Cell selection indicator after release of TCH and SDCCH” in        CHANNEL RELEASE in GERAN.    -   To keep the UE in the current RAT (for example GSM or WCDMA)

There is a desire to improve the performance in cellular radio networks.In particular there is a desire to improve performance in deploymentscenarios where many radio access technologies such as GERAN/UTRAN andE-UTRAN are deployed in the same area and where fallback to CS is used.

SUMMARY

It is an aspect of the present disclosure to provide an improved methodand device for keeping a preferred distribution of idle UE:s betweenGERAN/UTRAN and E-UTRAN.

This is obtained by the methods and devices as set out in the appendedclaims.

The decision whether the UE shall be kept in GERAN/UTRAN (also termed2G/3G), or moved back to E-UTRAN in a fast manner can in someembodiments be made by using explicit signaling in the radio accessnetwork taking into account e.g. the fact whether a CS call wasperformed as a result of a native CS fallback (CSFB) procedure, i.e. aprocedure described above by which the UE has been moved from theE-UTRAN coverage to 2G/3G coverage so the CS call can be performed.

When making such a decision the knowledge of whether the CS call isperformed due to CSFB or not as well as if it was a native CSFB ishelpful. In accordance with some embodiments, if the call has used aCSFB procedure, the mobile station is returned to E-UTRAN but if it thecall was originally setup in a 2G/3G cell with E-UTRAN coverage, themobile station is kept in 2G/3G.

It is also an aspect of the disclosure to provide methods andarrangements for informing a Base station Controller/Radio NetworkController (BSC/RNC) whether a CS call has used the CSFB procedure ornot.

These methods are described in more detail in the detailed description.

Thus, in accordance with some embodiments a method in a central node ofa cellular radio network the central node is provided where the centralnode is adapted to deciding if a mobile station connected to thecellular radio network via a first radio access technology controlled bythe central node is to be moved to another radio access technology whena circuit switched call of the mobile station is ended. It is assumedthat the cellular radio network is adapted to use a fallback to circuitswitched procedure. First it is decided that a CS call for a particularmobile station has ended. Then it is checked if the CS call was theresult of a fallback to CS procedure. The central node then decides ifthe mobile station is to be moved to another radio access technologybased at least partly on the determination, i.e. if the CS call was aresult of a CS fallback procedure.

For example if the circuit switched call was made by a mobile stationhaving performed a fallback to circuit switched procedure, the mobilestation can be returned to an original radio access technology fromwhich radio access the fallback to circuit switched procedure wasperformed. The original radio access technology can in accordance withsome embodiments be an evolved UTRAN or a similar radio accesstechnology not supporting a CS connection.

In accordance with some embodiments, the central node, which can be anRNC or a BSC receives information about if the circuit switched call hasresulted from fallback to circuit switch procedure or not in a messagefrom a Mobile Switching Center.

The disclosure also extends to nodes, such as a RNC or a BSC, and alsoto a Mobile Switching Center in a cellular radio system configured toperform the methods as described herein. The nodes can be provided witha controller/controller circuitry for performing the above processes.The controller(s) can be implemented using suitable hardware and orsoftware. The hardware can comprise one or many processors that can bearranged to execute software stored in a readable storage media. Theprocessor(s) can be implemented by a single dedicated processor, by asingle shared processor, or by a plurality of individual processors,some of which may be shared or distributed. Moreover, a processor or mayinclude, without limitation, digital signal processor (DSP) hardware,ASIC hardware, read only memory (ROM), random access memory (RAM),and/or other storage media.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail by way ofnon-limiting examples and with reference to the accompanying drawings,in which:

FIG. 1 shows the SAE/LTE architecture,

FIG. 2 illustrates the CS Fallback architecture,

FIG. 3 shows CS Fallback—Mobile Originating Call in active mode, PS HO,

FIG. 4 shows a PS HO from E-UTRAN to GERAN, preparation phase,

FIG. 5 is a view of an exemplary radio network,

FIG. 6 is a view of an exemplary mobile station,

FIG. 7 a view of an exemplary radio base station,

FIG. 8 a view of an exemplary central node, and

FIG. 9 is a flow chart illustrating some steps performed in a centralnode.

DETAILED DESCRIPTION

In the following description, describing some exemplary embodiments, twoscenarios are analyzed: A mobile station originated Circuit SwitchedFallback (CSFB) call respective a mobile station terminated CSFB call.In relation to these two exemplary scenarios it is here assumed thatCSFB capable Long Term Evolution/Wideband Code Division MultipleAccess/Global System for Mobile communication (LTE/WCDMA/GSM) mobileterminals in idle mode are distributed on a combined GSM/WCDMA/LTEnetwork based on some criteria, e.g. CS centric users are kept in GSMwhile less CS centric users primarily using Packet Switched (PS) dataservices are kept in LTE and/or WCDMA. Further to simplify thedescription the term mobile station will be used to refer to any deviceconnectable to a cellular radio network such as any User Equipment or(mobile) terminal.

In the GSM/WCDMA network this scenario implies that the CS speech callseither are ordinary mobile station originated/terminated CS speech callsinitiated in the GSM/WCDMA radio network or mobile stationoriginated/terminated CSFB calls initiated from the LTE or WCDMA radionetwork. If the distribution of the CSFB capable mobile stations is toremain the same in idle mode, the terminals that have performed a CSFBcall are advantageously directed back to the radio network from whichthe CSFB call was initiated after the CSFB call has ended.

To enable the network to return a mobile station to another radio accesstechnology the central nodes in GERAN or UTRAN, i.e. the Base stationController/Radio Network Controller (BSC/RNC), needs to have informationif a speech call was initiated using a CSFB procedure or not.

Mobile Station Originated Call:

In this scenario a mobile station performs a CSFB to a 2G or 3G network,i.e. a GERAN or UTRAN network, at a mobile originated (MO) CS call, i.e.a CS call initiated by a mobile station. The mobile station indicates tothe Mobile switching Center (MSC) in a 3GPP 24.008 CM SERVICE REQUESTmessage that this MO call is due to CSFB by setting a new InformationElement (IE) here termed CSMO. As a result the MSC is aware that thecall is due to CSFB, however, this message is transparent to the BSC/RNCand thus these nodes remain unaware.

In accordance with some embodiments a new Information Element on the Aand on the lu interface is added. This new IE can in accordance withsome embodiments be added to existing Base Station Subsystem (BSS)Management Application sub-Part/Radio Access Network Application Part(BSSMAP BSSMAP/RANAP) messages. For example, the BSC/RNC can receivefrom the MSC a new Information Element about whether the call being dueto CSFB added to the existing messages specified:

-   -   For the A interface in 3GPP TS 48.008:        -   ASSIGNMENT REQUEST        -   RELEASE COMMAND        -   COMMON ID        -   HANDOVER REQUEST    -   For the Iu interface in 3GPP TS 25.413:        -   RAB ASSIGNMENT REQUEST        -   IU RELEASE COMMAND        -   COMMON ID        -   RELOCATION REQUEST

Mobile Station Terminated Call

In this example the MSC is assumed to be aware of the call being due toCSFB as it has triggered the CSFB. Two different scenarios aredescribed.

In the first scenario the mobile station has been connected to thenetwork in E-UTRAN when paged by an MME. The MSC is aware that themobile station is registered over SGs and can use the same means asoutlined above to inform the BSC/RNC that the CS call is due to CSFB.

In the second scenario the mobile station has performed a combinedTracking Area update/Location Area Update (TAU/LAU), however, thereafterthe mobile station re-selected/moved to 2G/3G coverage. This case existsif S4-SGSN (Serving GPRS Support Node) is deployed and ISR is supportedin the network which is further discussed below. Otherwise a move to2G/3G typically will trigger RAU (Routing Area Update) and LAU orcombined RAU/LAU procedures.

Assume that the mobile station moves to the same Location Area (LA) asthe one that it has registered to by using combined TAU/LAU. The mobilestation will then perform RAU to obtain its PS services in the 2G/3G PSnetwork, however, it has not performed a new LAU procedure. The resultis that:

-   -   MSC has the valid registration associated with the MME by SGs        interface    -   MME is aware that the mobile station is registered to the 2G/3G        SGSN

Therefore the MSC establishing a CS call for the mobile station willpage the mobile station via the MME assuming that the mobile station isstill in the E-UTRAN coverage.

Given that the MME is aware about the mobile station now beingregistered to a specific 2G/3G SGSN, it will forward the paging messageto the appropriate 2G/3G SGSN. The paging procedure will be completed bythe 2G/3G SGSN using the existing procedure(s).

The mobile can access the MSC, i.e. send 3GPP TS 44.018 Page Response inresponse to the CS paging received via the SGSN in the exactly samemanner as if it has been paged via the MME.

Hence, the MSC will assume that this is a CSFB call, while the mobilestation was at the time of the paging procedure under 2G/3G coverage andthus the call is actually not a native CSFB. In other words theprocedure is not a procedure by which the mobile station has been movedfrom E-UTRAN coverage to 2G/3G coverage so that a CS call can beperformed. The information that the mobile station was actually under2G/3G coverage may be of importance in the algorithm implemented in thecentral node, BSC/RNC, that decides how the mobile station at therelease of the CS call shall be managed with respect to the inter RATscenario.

Though the procedure described above may be acceptable also for the casewhen the call is terminated by the mobile station it can in someembodiments be beneficial if the BSC/RNC is configured to distinguishbetween a circuit switched fallback scenario when the call is terminatedby the mobile station where the mobile station is connected via an LTEradio access and when it is connected via a 2G/3G radio access.

In accordance with some embodiments the Gb/Iu_PS interface is modifiedin such a way that the BSC/RNC can distinguish that a CS paging messageis received from the MME via the S3 interface. In accordance with oneembodiment this is achieved by adding e.g. a new Information Element ora new flag to an existing Information Element, appended to the CS Pagingmessage.

The BSC/RNC can in some implementations then cache such CS Pagingmessage and store the included International Mobile SubscriberIdentity/Temporary Mobile Subscriber Identity (IMSI/TMSI) value toidentify the CS calls indicated by using the same procedure as suggestedabove for mobile originating call as CSFB as whether this CS call wasinitiated while the mobile station was under LTE or 2G/3G coverage.

An alternative implementation is to use the fact that in the case of acircuit switch fallback for a mobile station terminated call, where themobile station that is subject for paging camps on LTE, the MME alwayssends the SGs SERVICE REQUEST message to the MSC. However, this messageis not sent from the MME if the mobile station camps, during the circuitswitch fallback for a mobile station terminated call procedure, on2G/3G.

Thus the reception of the SGs SERVICE REQUEST message can be used by theMSC to decide whether a CSFB indication as described for mobile stationoriginating calls above shall be sent to the BSC/RNC or not inASSIGNMENT REQUEST/RAB ASSIGNMENT REQUEST.

EXAMPLE IMPLEMENTATIONS

Although the described solutions may be implemented in any appropriatetype of telecommunication system supporting any suitable communicationstandards and using any suitable components, particular embodiments ofthe described solutions may be implemented in an LTE network, such asthat illustrated in FIG. 5.

As shown in FIG. 5, the example cellular radio network 11 may includeone or more instances of mobile stations 13 and one or more basestations 12 capable of communicating with these mobile stations, alongwith any additional elements suitable to support communication betweenmobile stations or between a mobile station and another communicationdevice (such as a landline telephone). Although the illustrated mobilestations may represent communication devices that include any suitablecombination of hardware and/or software, these mobile stations may, inparticular embodiments, represent devices such as the example mobilestation illustrated in greater detail by FIG. 6. Similarly, although theillustrated base stations may represent network nodes that include anysuitable combination of hardware and/or software, these base stationsmay, in particular embodiments, represent devices such as the examplebase station illustrated in greater detail by FIG. 7.

As shown in FIG. 6, the example mobile station 13 includes a processor111, a memory 113, a transceiver 112, and an antenna 23. In particularembodiments, some or all of the functionality described above as beingprovided by mobile communication devices or other forms of mobilestation may be provided by the mobile station processor 111 executinginstructions stored on a computer-readable medium, such as the memory113 shown in FIG. 6. Alternative embodiments of the mobile station mayinclude additional components beyond those shown in FIG. 6 that may beresponsible for providing certain aspects of the mobile station'sfunctionality, including any of the functionality described above and/orany functionality necessary to support the solution described above.

As shown in FIG. 7, the example base station 12 includes a processor101, a memory 103, a transceiver 102, and an antenna 108. The examplebase station can also comprise a network interface 104. In particularembodiments, some or all of the functionality described above as beingprovided by a base station, a base station controller, a node B, anenhanced node B, and/or any other type of mobile communications node maybe provided by the base station processor executing instructions storedon a computer-readable medium, such as the memory shown in FIG. 7.Alternative embodiments of the base station may include additionalcomponents responsible for providing additional functionality, includingany of the functionality identified above and/or any functionalitynecessary to support the solution described above.

Further, FIG. 8 depicts an exemplary central node 20, such as an RNC ora BSC. The example central node 20 includes a processor 201, a memory203, and a network interface 204 for connection to other nodes of acellular network such as a base station and a Mobile Switching Center.In particular embodiments, some or all of the functionality describedabove as being provided by a central node, may be provided by theprocessor 201 executing instructions stored on a computer-readablemedium, such as the memory 203.

In accordance with some embodiments, the steps as illustrated in FIG. 9can be performed by a central node. Thus in FIG. 9 a flow chartillustrating some steps performed in a central node such as a RadioNetwork Controller (RNC) or a Base Station Controller (BSC) is shown.Thus, in a method in a central node of a cellular radio network thecentral node is adapted to deciding if a mobile station connected to thecellular radio network via a first radio access technology controlled bythe central node is to be moved to another radio access technology whena circuit switched call of the mobile station is ended. It is assumedthat the cellular radio network is adapted to use a fallback to circuitswitched procedure. First in a step 901 it is decided that a CS call fora particular mobile station has ended. Then in a step 903 it is checkedif the CS call was the result of a fallback to CS procedure. The centralnode then in a step 905 decides if the mobile station is to be moved toanother radio access technology based at least partly on thedetermination in step 903, i.e. if the CS call was a result of a CSfallback procedure.

For example if the circuit switched call was made by a mobile stationhaving performed a fallback to circuit switched procedure, the mobilestation can be returned to an original radio access technology fromwhich radio access the fallback to circuit switched procedure wasperformed. The original radio access technology can in accordance withsome embodiments be an evolved UTRAN or a similar radio accesstechnology not supporting a CS connection.

In accordance with some embodiments, the central node receivesinformation about if the circuit switched call has resulted fromfallback to circuit switch procedure or not in a message from a MobileSwitching Center.

The invention claimed is:
 1. A method in a central node of a cellularradio network that is adapted to use fallback to circuit switchedprocedures, the method comprising: determining whether or not a circuitswitched call of a mobile station connected to the cellular radionetwork via a first radio access technology controlled by the centralnode has resulted from a fallback to circuit switched procedure; anddeciding whether the mobile station is to be moved to another radioaccess technology when the circuit switched call of the mobile stationis ended, wherein said deciding is based on whether or not the circuitswitched call has resulted from a fallback to circuit switchedprocedure.
 2. The method of claim 1, wherein if the circuit switchedcall has resulted from a fallback to circuit switched procedure, themobile station is returned to an original radio access technology fromwhich radio access technology the fallback to circuit switched procedurewas performed.
 3. The method of claim 2, wherein the original radioaccess technology is an evolved UTRAN.
 4. The method of claim 1, whereinthe central node is a Base Station Controller or a Radio NetworkController.
 5. The method of claim 1, wherein the central node receivesinformation about whether or not the circuit switched call has resultedfrom a fallback to circuit switched procedure in a message from a MobileSwitching Center.
 6. A central node of a cellular radio network that isadapted to use fallback to circuit switched procedures, said centralnode comprising a controller configured to: determine whether or not acircuit switched call of a mobile station connected to the cellularradio network via a first radio access technology controlled by thecentral node has resulted from a fallback to circuit switched procedure;and decide whether the mobile station is to be moved to another radioaccess technology when the circuit switched call of the mobile stationis ended, wherein said deciding is based on whether or not the circuitswitched call has resulted from a fallback to circuit switchedprocedure.
 7. The central node of claim 6, wherein the controller isconfigured to return the mobile station to an original radio accesstechnology from which radio access technology a fallback to circuitswitched procedure was performed if the circuit switched call resultedfrom a fallback to circuit switched procedure.
 8. The central node ofclaim 7, wherein the original radio access technology is an evolvedUTRAN.
 9. The central node of claim 6, wherein the central node is aBase Station Controller or a Radio Network Controller.
 10. The centralnode of claim 6, wherein the central node is configured to receiveinformation about whether or not the circuit switched call has resultedfrom a fallback to circuit switched procedure in a message from a MobileSwitching Center.
 11. A method in a mobile switching center of acellular radio network that is adapted to use fallback to circuitswitched procedures, the method comprising: determining whether or not acircuit switched call has resulted from a fallback to circuit switchedprocedure; and transmitting, to a central node of the cellular radionetwork, a message comprising information about whether or not thecircuit switched call has resulted from a fallback to circuit switchedprocedure.
 12. The method of claim 11, wherein the central node is aBase Station Controller or a Radio Network Controller.
 13. A mobileswitching center for a cellular radio network that is adapted to usefallback to circuit switched procedures, wherein said mobile switchingcenter is configured to: determine whether or not a circuit switchedcall has resulted from a fallback to circuit switched procedure; andtransmit, to a central node of the cellular radio network, a messagecomprising information about whether or not the circuit switched callhas resulted from a fallback to circuit switched procedure.
 14. Themobile switching center of claim 13, wherein the central node is a BaseStation Controller or a Radio Network Controller.